1. Field of the Invention
The present invention relates to a liquid developer for developing electrostatic latent images formed on the surface of a latent image carrying member, said liquid developer comprising a dispersion of specific polymer particles in a dispersion medium.
2. Description of the Related Art
Electrophotographic methods can be broadly divided into dry process developing methods and wet process developing methods. Among these methods, developing methods of the wet process type use toner dispersed in a fluid, and a toner particle diameter to the submicron range can be achieved. Having a toner particle diameter in this range is advantageous inasmuch as it produces high resolution with excellent halftone qualities, and readily allows fixing of the toner image.
In conventional developing processes of the wet process type using toner particles in the submicron range require a high electric field to achieve each electrostatic transfer, thus making said transfer difficult due to excessive charge of the toner. As the toner particle diameter becomes smaller, there is less movement of the toner in the fluid, resulting in the disadvantage of slower developing speed which makes it unsuitable for high-speed applications.
Liquid developers using relatively large size polymer particles as the toner particles so as to eliminate the aforesaid disadvantages are subject to loss of dispersion stability, and polymer particle sedimentation.
In general, there are various theoretical equations which express particle sedimentation rates, but the best known is the Stokes equation. Actually, the sedimentation of particles in a fluid is dependent on various factors such as particle shape, charge and the like, but when assuming the sedimentation of spherical particles in an infinite Newtonian fluid influenced by gravity alone, the particle sedimentation rate is expressed by Equation 1 below. EQU V=218r.sup.2 (P.sub.s -P)/.eta. (1)
where the following values obtain:
V: particle sedimentation rate PA1 r: particle size PA1 P.sub.s : particle density PA1 P: solvent density PA1 .eta.: solvent viscosity
As can be understood from the above equation, in general, the larger the particle size, the greater the sedimentation rate and the more readily sedimentation occurs. Thus, using large size toner in a liquid developer can be said to be advantageous inasmuch as developing speed and transfer efficiency are improved, while also being disadvantageous insofar as long-term stability and storage characteristics are adversely affected.
When particles settle, the liquid developer density changes so as to adversely affect the image and cause an erroneous warning that it is time to replenish the toner, i.e., of the existence of a "toner empty" condition. Although this situation may not occur if the liquid developer is restored to its original state by a simple mixing mechanism when a slight amount of particles settle, the practicality of such a solution is reduced when a large scale mixing mechanism or resupply mechanism is required to simply re-disperse the settled particles.